The Rotator Cuff – What is it?

Shoulder Anatomy

The rotator cuff is a group of four muscles that make up part of your shoulder (see Figure 1). These muscles each attach to the shoulder blade (scapula) at one end, and the top of the arm (humerus) at the other.

Function of the Rotator Cuff

[…] the rotator cuff’s primary role is to keep the arm centred in the shoulder socket (glenoid cavity). All four muscles act as a unit to pull the arm into the centre of the shoulder socket during movement. Together with the deltoid, this creates a “force couple”, allowing motion to be created by rotation, rather than a change of position (Figure 2)*. This is highly important when we use our arms because the shoulder socket is very shallow — a shallow socket allows a wide range of movement at the shoulder, but does little to prevent dislocations! So without something consistently pulling the arm back to the centre of the shoulder, we would be in big trouble!

Consider raising your arms overhead at the side (like making a snow-angel). As your arms leave the side of your body, the four rotator cuff muscles work as a unit, pulling inward towards your centre. Without them, the other muscles at your shoulder (trapezius, deltoid) would draw the arm up and out of the shoulder socket, likely dislocating your arm!

Figure 2: A partial example of compression at the shoulder

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Why does the rotator cuff matter to me?

Since nearly every movement of the arm requires the rotator cuff muscles to hold things in the middle, many shoulder pains or dysfunctions involve the rotator cuff. If just one of the muscles works too little, too much, or out of sync with the others, the entire unit is jeopardized.

Occasionally, this results in damage to the muscle(s). Many people experience “rotator cuff tears”, either partial (a little bit of damage) or full (the whole muscle is torn), requiring treatment to limit pain, increase function, and rehabilitate the shoulder. Usually this is the result of some activity or incident that created the damage — this is a “traumatic” rotator cuff tear. This is probably more common than you might think — in fact,Minagawa et al. (2013) examined 3000 people (ages 20-90) from a single village, finding full thickness tears in 0-36% of people by decade (Figure 3). So once you reach 50 years of age, at least 1 in 10 people have a full thickness tear.

Conversely, where there is no activity or incident leading to shoulder discomfort, we also must consider “age-related” rotator cuff tears. We see many instances where people experiencing no symptoms at all (“asymptomatic” people) display rotator cuff tears when they are assessed by advanced imaging (ultrasounds, MRIs, etc). The number of these cases appears to go up as a person’s age increases. For example, Tempelhof et al. (1999) examined people over the age of 50 and identified asymptomatic rotator cuff tears in 13-51% of the sample (see Figure 4a). This was supported by the Minagawa study as well, who found 50% of tears in people’s 50s were asymptomatic, and at older ages this increased even more, with asymptomatic tears significantly outnumbered symptomatic tears (>50%) (see Figure 4b).

So is having a rotator cuff tear a problem?

That really depends on your individual case. Sometimes rotator cuff tears are asymptomatic — you have no obvious signs of pain, diminished function, or limited activities, and yet a tear is present if you get advanced imaging of the shoulder. Other times tears are symptomatic, causing pain, limiting function, and getting in the way of your activities, and getting treatment to heal, repair, and rehabilitate the shoulder is extremely helpful. Conservative treatments (exercise, manual therapy, etc) have been shown to provide beneficial outcomes for people with symptomatic tears (Ainsworth et al. 2007; Kuhn et al. 2009), so combining interventions like soft tissue therapy, physical modalities, and exercise is often a good place to begin treatment.

In Summary

The rotator cuff is extremely important for typical activities involving the shoulder, providing constant compression which allows our extreme freedom of movement without risking dislocation. However, the rotator cuff is used so frequently in our lives that damage ultimately occurs in a high proportion of the population as we age. Fortunately, this damage does not always create pain or limit function, and when it does manual treatments have good success at resolving the issues.